Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 23, 2015
Valid: July 24 – 28, 2015 (Friday – Tuesday)

Summary:

An upper level ridge over the southern US will dominate the weather for the entire medium range period, leading to clear skies, light winds, and Marginal chances for USG ozone Saturday-Tuesday. Ozone and PM2.5 concentrations will gradually increase throughout the period. Monday and Tuesday will be increasingly hot and humid, which will enhance the chances for rising ozone. There is uncertainty in the precipitation forecast for the end of the period, however. The wetter GFS would bring clouds and precipitation to most of the region, limiting ozone production.

Discussion:

The weather models are in close consensus throughout the medium range period. The 06Z NAM, 00Z GFS, and 00Z ECMWF were consulted for this analysis. Currently, a broad upper level ridge over the southern US, with its axis over the central Plains, is dominating the weather for most of the CONUS. The axis of this ridge will move eastward on Friday, reaching the Ohio River Valley (ORV) by 00Z Saturday. The location of the upper level ridge translates to surface high pressure centered over the OH/IN border. On Saturday, the axis of the upper level ridge will move over the Mid-Atlantic as a closed upper level low and accompanying shallow trough over ON move eastward. Shortwaves embedded in the base of this trough will graze the northern Mid-Atlantic (NMA) Saturday evening into Sunday morning. A cold front associated with the shallow trough will move eastward as well on Saturday; however, it is expected to dissipate on Sunday as the closed low over ON moves northward towards James Bay and occludes. The southern US ridge will strengthen on Sunday and amplify its axis over the central Plains. This ridge will dictate the weather for the rest of the medium range period. The weather models hint at bringing some weak shortwaves over the crest of the ridge and into the Mid-Atlantic on Sunday, Monday, and Tuesday. The biggest impacts of these shortwaves will probably be felt on Tuesday when the chances for clouds and precipitation are the highest.

Surface high pressure will dominate the weather on Friday for most of the Mid-Atlantic leading to clear skies and light northwesterly winds. A fairly clean background air mass in place over the region will limit substantial ozone production, despite the clear skies. Locations east of the I-95 Corridor have the best chances to see Moderate ozone production. The air quality models are showing Moderate ozone in southern MD and eastern NJ, likely in response to the northwesterly surface winds. In NC, clouds, showers, and thunderstorms associated with a surface wave along a stalled frontal boundary will keep ozone in the Good range on Friday. Low humidity and light surface winds will limit particle concentrations to the upper Good range on Friday. A few areas in NC along the stalled frontal boundary where precipitation is the lightest may see particles reach the low Moderate range.

On Saturday, high pressure will move over the Mid-Atlantic, leading to mostly clear skies and light and recirculating surface winds. Temperatures in the upper 80s F will create conditions favorable for ozone production. Moderate ozone is likely in areas around the I-95 Corridor and NJ. The air quality models are showing mid to upper Moderate ozone in these areas. However, scattered clouds and light showers in PA Saturday afternoon will limit ozone from reaching high into the Moderate range. The light surface winds coupled with a rebound in humidity will increase PM2.5 concentrations, but only to the Good/Moderate threshold.

A weak cold front will reach western PA Sunday morning, leading to scattered clouds and showers in the NMA. This front will dissipate into a surface trough as it moves eastward through PA. Surface high pressure will keep skies clear in the central Mid-Atlantic (CMA) and SMA. Light winds coupled with the clear skies and recirculating back trajectories will promote rising ozone, especially in areas along the I-95 Corridor. Lower Sunday emissions should cap ozone production, limiting concentrations to the Moderate range. The air quality models are showing mid to upper Moderate ozone along the I-95 Corridor and in NJ. PM2.5 concentrations will also continue to rise, just into the Moderate range, due to the light winds and increasing humidity.

Monday and Tuesday will be the hottest and most humid days of the period. Surface high pressure will continue to promote clear skies and light winds for most of the region. As a result, rising ozone and PM2.5 concentrations will be a threat. The dissipating cold front will act as a line of convergence, allowing for a buildup of pollutants. A limiting factor may be the upper level transport, which will be northerly from southeastern ON. In addition, there is still uncertainty in the precipitation forecast for Monday and Tuesday. Upper level shortwaves moving overhead may lead to clouds and precipitation in the CMA and SMA. The GFS is the wetter solution, compared to the drier EC solution, but there is enough model divergence to make the precipitation forecast uncertain.

-DeBoe/Huff